The present invention relates to the CT imaging arts. The invention finds particular application in conjunction with real time continuous CT imaging and will be described with particular reference thereto. However, it is to be appreciated that the present invention will also find application in conjunction with other types of CT imaging apparatus and techniques, as well as other diagnostic imagers.
Early CT scanners were of a traverse and rotate type. That is, a radiation source and oppositely disposed radiation detector traversed together along linear paths on opposite sides of a subject. The detector was repeatedly sampled during the traverse to create a plurality of data values representing parallel rays through the subject. After the traverse, the entire carriage was rotated a few degrees and the source and detector were traversed again to create a second data set. The plurality of parallel ray data sets at regular angular intervals over 180.degree. were reconstructed into a diagnostic image. Unfortunately, the traverse and rotate technique was very slow.
One technique for speeding traverse and rotate scanners was to replace the radiation source and single detector with a radiation source that projected radiation along a narrow fan beam and to provide several detectors such that a plurality of parallel ray data sets at different angles were collected concurrently. In this manner, several of the data sets could be collected concurrently. This was several times faster, but still very slow.
Rather than traversing the source and detector, it was found that the radiation source could be rotated only. That is, the radiation source projected a fan of data which spanned the examination region. An arc of radiation detectors received the radiation which traversed the examination region. The radiation source was rotated around the subject. In a third generation scanner, the arc of detectors rotated with the source. In a fourth generation scanner, an entire ring of the stationary detectors was provided. In either type, fan beam data sets were sampled at a multiplicity of a apexes around the subject. The data from the different angles within the fans at different angular orientations of the fan were sorted into parallel ray data sets. It was found that a complete set of parallel ray data sets could be generated by rotating the source 180.degree. plus the fan angle. Although much faster than the traverse and rotate technique, a larger amount of data processing was required to sort or rebin the rays into the parallel ray data sets and to interpolate, as necessary, in order to make the rays within each data set more parallel. Although the data collection time was much faster, the image processing was slower. Parallel beam reconstruction was abandoned largely due to memory and speed limitations, as well as due to inaccuracy of the rebinning step when performed early in the processing chain without accurate detector corrections and angular view filtering, especially when a limited number of views were collected.
Rather than sorting the data into parallel ray data sets, it was found that the fan beam data sets could be reconstructed directly into an image representation by convolution and backprojection. Although the convolution and backprojection technique required significantly less processing hardware and time than the rebidding technique, the data collection was slower. In particular, the algorithm required the apexes of the data fans to span a full 360.degree., not just the 180+ fan angle.
More recently, improved convolution and backprojection techniques in which the apexes of the data fans need only span 180.degree. plus the fan angle have been developed. While today these techniques are among the most widely used CT reconstruction algorithms, they still have drawbacks. In particular, they can be computationally complex and time consuming.
While successful for their intended uses, the previously described CT scanners and techniques have inherent drawbacks which make them unsuitable for real time imaging. In particular, the various combinations of data collection and data processing techniques are too time consuming to allow for accurate continuous image updating in real time.
The present application contemplates a new and improved CT scanner which overcomes the above-referenced problems and others.